A variety of fabrication techniques may be utilized to create devices with microfluidic channel structures. Generally, a microfluidic channel is a channel defined by a material and having a channel dimension of 1 mm or less. Microfluidic channels may be defined by a variety of materials including etched glass, silicon, molded polydimethylsiloxane (PDMS), and molded or embossed plastic. The microchannels themselves may be formed by etching away portions of the material, by embossing, or by building up a microchannel structure through the deposition of patterned layers. The material used and channel formation technique may be chosen based on the cost of fabrication and whether the resultant device will have the feature dimensions and material compatibility desired in a particular application.
Devices made using etching processes are generally costly, but may achieve accurate dimensions and be well-suited to a variety of applications. PDMS and plastic devices may be less costly but less well defined and may have heterogeneous chemical properties subject to temporal variation within the device. In addition, PDMS, as a porous material, may be prone to reagent evaporation through the material and may be unsuitable for long term device storage.
It may be desirable to control one or more interior surfaces of microchannels to be able to control the surface charge, hydrophobicity, binding sites, or other material linkages to the microchannel surface. Methods for controlling the surface reactivity of PDMS and plastic microchannels may not be as well developed as those for glass surfaces that can be treated by self assembly of silane monolayers. For example, a reactive surface chemistry may be needed to enable the later attachment of other materials or structures, such as polyacrylamide gels. For some applications in plastic, plasma surface treatments may be used to modify the chemistry of surfaces. However, the plasma surface treatments may result in a low density of reactive chemical groups, heterogeneous and undefined reactive groups, and non-uniform surface coverage of reactive groups, which may be problematic. In addition, undesired surface properties such as introduction of non-reactive charged groups during surface treatment can be problematic. For example, these surface treatment problems may result in undesired electroosmotic fluid flow or undesired chemical interactions with fluid constituents in some cases.
Examples of forming polymeric microdevices using photopolymerization is described in U.S. Pat. No. 6,488,872 entitled “Microfabricated devices and method of manufacturing the same,” which patent is hereby incorporated by reference in its entirety for any purpose. A liquid phase monomer solution including a photoinitiator may be loaded into a cartridge, and microchannels defined by polymerizing regions of the solution by exposing them to UV light. U.S. Pat. No. 6,488,872 further describes the later polymerization of hydrogels in the microchannels.
In a variety of applications, it may be desirable to place polyacrylamide gels at one or more locations in a microchannel. The polyacrylamide gels may be used as size-exclusion membranes, concentration membranes, or binding sites for analytes. The polyacrylamide gels are generally formed by photopolymerizing a polymer solution. On exposure to light, the polymer solution is polymerized into a gel. However, in some cases, the hydrogel may not adhere adequately or at all to one or more the microchannel surfaces. The poor adhesion may be due to surface chemistry difficulties described above. Consequently, flow through the microchannel may be disrupted in the region of a partially adhered gel, and bubbles or unusual streaming patterns may result, disrupting the proper operation of the microchannel and gel. In some cases, it may not be feasible at all to attach the gel.
Examples of polymeric microdevices are also described in WO 2004/009489 entitled “Fabrication of 3D photopolymeric devices,” which publication is hereby incorporated by reference for any purpose. Iniferters or iniferter precursors are added to the liquid phase monomer solution used to form microchannels to allow covalent bonding of subsequent layers of material to a polymer surface. Iniferters are incorporated into a liquid polymer composition used to define one or more microstructures prior to polymerization. Once polymerized, the resulting polymeric layer contains iniferter on the surface of the material which facilitates bonding of subsequent layers to the polymerized surface. In this manner, a microchannel surface may be formed having iniferters and a photopolymerized hydrogel may be adhered to the microchannel surface. The described systems utilize a photomask as an upper layer of the microchannel system during formation which is removed following polymerization, resulting in open microchannels which are later sealed. A chuck system is also described that may be used to set the thickness of the polymer layer between the photomask and the substrate on the chuck.